This invention relates to a process for determining the wheel slip of individual wheels of a vehicle by intermittentently interrupting the application of braking force to each wheel of the vehicle and comparing the speed of each such wheel with braking force applied, to the speed of the same wheel without braking force applied.
A process for determining the slip of a wheel by measuring its speed during braking and comparing it with the speed of a separate reference wheel running unbraked is already known to the art, and is well exemplified by German Patent Document No. DE-2,334,235-B2. In this arrangement, the reference wheel is one of the rear wheels. The braking force on the reference wheel is interrupted cyclically, the time intervals between these interruptions being dependent on vehicle speed and the effective braking force. The slip of each individual wheel of the vehicle is determined by comparing its speed during braking with the speed of the unbraked reference wheel.
However, the use of a separate reference wheel to determine the slip of the individual wheels in this manner introduces several sources of error into the process. In particular, the difference between the radii of the tested wheel and of the reference wheel is neglected. Moreover, additional error arises because the radius of the curvature to be negotiated is generally different for each wheel, causing each wheel to have a different rotational speed when negotiating a curve in the unbraked state. Consequently, in order to accurately determine the slip of the individual wheels by using one wheel as a reference wheel, cornering would have to be taken into account separately.
The object of the present invention is to improve the known process in such a way that the slip .sigma. of the individual wheels of a vehicle can be determined precisely within the stable braking range, without the vehicle's assuming an unstable driving state as a result of the determination of the slip during braking.
Additional objects of the invention are:
to provide a process for determining the braking-force coefficients of the individual wheels of a vehicle;
to permit the determination of specific adhesion utilization for different grips of the individual wheels on the road surface;
to establish a specific steering characteristic for cornering during braking; and
to provide a process to achieve a uniform wear of brake linings.
Furthermore, by the use of a combination of one or more suitable sensors such as, for example, a sensor for measuring the brake pressure applied to the individual wheels to determine the braking force F.sub.b, a deceleration sensor and a sensor for measuring the total weight of a vehicle to determine the braking force F.sub.B and/or a transverse-acceleration sensor, it is thus possible, by precise determination of the wheel slip in the manner set forth and described herein, to obtain a specific driving behavior during braking as a result of a suitable brake-pressure distribution, and/or to diagnose the state of the wheels. The use of further suitable sensors is described in more detail hereafter where appropriate.
In general, the slip .sigma. of a wheel is obtained by comparing the speed of the free-running wheel of a vehicle n.sub.f with the speed of a braked wheel n.sub.b according to the following formula: ##EQU1## In the process according to the invention, the slip of the individual wheels is measured precisely during a braking operation in the stable braking range, by intermittent interruption of the brake pressure on each wheel so that such wheel rotates at the speed of a free-running wheel. In one embodiment of the invention, the length of the period of time in which the brake pressure is thus removed from each wheel amounts to approximately 0.1 second, and the speed of each wheel in its free-running state is measured at the end of this period of time. In a second embodiment, an improved process for detecting the state of the free-running wheel is obtained by comparing the rotational acceleration of the wheel with a predetermined threshold value. The state of the free-running wheel is detected when the rotational acceleration of the wheel has fallen below a predetermined threshold value. The braking force is subsequently applied again, and the speed of the braked wheel is again measured when the rotational deceleration of the wheel has fallen below a predetermined threshold value. Wheel slip is then determined by comparison of the speed of the wheel in its free running state and in its braked state.
Since a sufficient amount of measured values must be available in order to provide accurate results during a braking operation, the above process of removing the brake pressure on each wheel is repeated at a time interval of a few tenths of a second. To prevent such periodic temporary interruption of the brake pressure from exciting natural vibrations of the vehicle, the time intervals between the individual interruptions of the brake pressure are varied. Moreover, provision can also be made for reducing the brake pressure simultaneously on all the wheels of an axle, in order to prevent undesirable yawing moments about the vertical axis of the vehicle.
One aspect of the present invention which must be taken into account is that, absent a compensating adjustment in the braking process, the braking distance of the vehicle would be lengthened as a result of the time-dependent interruption of the brake pressure, compared with a vehicle in which no such interruption of the brake pressure is carried out. Thus, in order to ensure that the braking behavior of the vehicle perceptible by the vehicle driver is not affected, and in particular to ensure that possible overreactions of the vehicle driver will be prevented, provision is made for determining a desired value for the intensity of the braking operation from the position of the brake-value transmitter of the brake system, and for achieving an overall braking force consistent with the desired value. Where appropriate, the determination of the desired value includes the vehicle speed as an additional parameter. That is, when the brake pressure is intermittentently interrupted by the control device during a braking operation, then, on the basis of the desired value determined, the brake pressure, when reapplied, is brought to a higher value than that corresponding to the value of the brake pressure when there is no time-dependent interruption of the brake pressure. The higher brake-pressure value is selected such that the time average of the braking force becomes equal to the time average of the braking force obtained when the brake pressure is continuously applied.
On a vehicle without an anti-lock system (ABS), the stable braking range is detected by the vehicle driver's predetermining a low desired value for the intensity of the braking operation. If there is nevertheless a locking of the wheels because of a low .mu.-value, the intermittent interruption of the braking force is not adversely affected since the braking force is thereby interrupted in such a way that the wheels of the vehicle start to run again; that is to say come nearer to the stable braking range. Moreover, the stable braking range can be detected from the fact that the wheel deceleration remains below an appropriately predetermined threshold value. To have a sufficient reserve in terms of the stability of the braking operation, this threshold value of the wheel deceleration is preferably selected so that it is below the response threshold of an ABS possibly present.
On a vehicle with ABS, the leaving of the range of the stable braking operation can additionally be detected by the presence of activating signals from the ABS to the pressure-reducing components of the brake system. In principle, a combination of these criteria is also possible for the purpose of detecting the leaving of the stable braking range. When the leaving of the stable braking range has been detected, no further output signals are transmitted by the control device.
Because the basic functional relationship between the braking force coefficient .mu. of a wheel relative to the road surface and the slip of such wheel .sigma. is known, the precisely determined wheel slip of the individual wheels of a vehicle determined in accordance with the invention can also be used to determine a breaking force coefficient for each wheel, thereby permitting detection of differences in the adhesion of each wheel to the road surface, and adjustment or compensation therefor.
The braking force coefficient .mu. can be determined separately for each wheel when the contribution F.sub.b of each wheel to the total effective braking force F.sub.B and also the gravitational force N.sub.g acting on this wheel is known. The braking force coefficient .mu. is derived as follows: ##EQU2##
The basic trend of the .mu./.sigma.-characteristic is known. This characteristic varies, on the one hand, with the conditions on the road surface, such as, for example, black ice, road wet from rain or dry road, and, on the other hand, with the properties of the wheel, such as, for example, wear, air pressure in the tire and the properties of the tire material. For this reason, a family of characteristics for the braking force coefficient .mu. against the wheel slip .sigma. is therefore obtained. The parameter for this variation in the characteristic is referred to below as the grip of the wheel with the road surface. Now since the braking force coefficient .mu. and the wheel slip .sigma. are determined individually for each wheel, it is possible for the basic grip of the wheel with the road surface to be determined individually for each wheel from the family of characteristics.
The contribution F.sub.b of an individual wheel to the total effective braking force F.sub.B can be determined by ascertaining the force which is exerted on this wheel by the brake system. This can be accomplished by direct measurement of the force on the brake caliper or by measuring the pressure on the wheel-brake cylinder, which is proportional to the braking force F.sub.b in close approximation. Furthermore, the total effective braking force F.sub.B can be determined from the total mass M of the vehicle and from the deceleration during the braking operation according to the formula: EQU F.sub.B =M*a
A determination of the braking force F.sub.b which is less exact under specific driving conditions may also be obtained if it is assumed that the vehicle is in a stable braking range such that all the wheels of the vehicle make the same contribution F.sub.b to the resultant braking force F.sub.B. The quantity F.sub.b is then calculated by dividing the quantity F.sub.B by the number of wheels of the vehicle. In this process for determining the braking force F.sub.b, there is no need for sensors which measure this braking force F.sub.b directly, but at the same time this determination of the braking force F.sub.b has the disadvantage of being less accurate than as if the braking force F.sub.b is determined directly by suitable sensors.
If the gravitational force N.sub.g acting on a wheel is known, the braking force coefficient .mu. can be determined according to the formula given above. The gravitational force N.sub.g acting on a wheel can be determined, for example, by means of wire strain gauges which are arranged at suitable locations on the axles in a known manner or, on air-suspension vehicles, also by measuring the pressure on the pneumatic spring bellows.
The state of the wheels and/or the nature of the road surface can be found from the determined values of the grip of the individual wheels with the road surface. In a simple form of evaluation, if there are different grip values on the two sides of the vehicle it is concluded that there is a ".mu.-split" condition. In this case, the grip of the wheels on the respective same side of the vehicle must assume the same value. If it happens that the grip of two or more wheels located on the same side of the vehicle is different, then it must be expected that the wheels will have different properties; that is to say that one of the wheels has a defect. If is it found over a relatively long period of time that the grip of the wheels is different on different sides of the vehicle, from which a .mu.-split condition has been concluded, when considered instantaneously, a defect of at least one of the wheels is likewise established. The expression "relatively long period of time" is intended to mean that at least several braking operations over a distance of several kilometers are to be taken into consideration. To avoid incorrect conclusions, this period of time should extend over several trips on several days.
If, on one of the wheels, an impaired grip with the road surface attributable to the wheel is detected, this defect of the wheel is communicated to the vehicle driver, either by indicating the defect qualitatively, without the indication containing information on the magnitude of the defect, or by giving the vehicle driver a signal which provides information on the magnitude of the defect. In a simple embodiment, the strength of the signal increases in proportion to the magnitude of the defect. In a preferred embodiment, the strength of the signal increases progressively with the magnitude of the defect of the wheel. Thus, the vehicle driver is warned increasingly clearly when a dangerous state of the wheel is being reached.
In the process according to the current invention, when a .mu.-split condition is detected, braking behavior can be controlled in a manner comparable to the known method of attenuating yawing moment, by means of a time-delayed pressure build-up in an ABS. The ABS exerts control only when the threshold values specific to the system are reached.
Moreover, under some circumstances, it is expedient to influence the steering characteristic of the vehicle during the negotiation of a curve and a simultaneous braking operation. For this purpose, whether the steering movement is to be assisted or weakened by the distribution of the brake pressure depends on the natural dynamics of the vehicle. The state of the negotiation of a curve is detected in a known manner by a transverse-acceleration sensor and/or a sensor for detecting the steering-lock angle in conjunction with a driving-speed sensor. The distribution of the braking force can be determined from a stored family of characteristics of brake pressure as a function of one or more of the quantities of transverse acceleration, .mu.-value, vehicle speed and desired value for the braking force to be applied. In a braking operation, since the lateral guiding force of the wheels decreases with increasing braking force, in general the brake pressure is distributed in such a way that the steering movement of the vehicle is assisted increasingly with an increase of one or more of the quantities of transverse acceleration, vehicle speed and desired value for the braking force to be applied. A decrease of the .mu.-value causes the vehicle to "push on the wheels". At a decreasing .mu., therefore, the brake pressure will be distributed in such a way that the steering movement of the vehicle driver is assisted in order to allow a better track-holding of the vehicle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.